I am going to bet you a big fat steak (or gallon of gas) you are on the wrong track.
GAT is not the maker of the tool.... it is the business that used/owned the tool. Stamped their initials and factory location to prevent theft/loss.
I have a very similar tool we used in the 50's and 60's to mic out flat stock and structural steels used in manufacturing. Highly expensive back in the day, coveted, and prized versus today where every Tom/Dick and Harry has tools like this in their loading rooms, shops, tool boxes, etc...
Was your Stepdad a welder/fabricator/engineer?
A History of Forward Thinking, Satisfied Clients and Long-Term Employees
Born out of engineering expertise and passion for efficient design, GAT was founded by the current owner’s grandfather in 1958 in St. Louis, Missouri. From the beginning, we put our focus on creating custom finishing solutions and process lines for companies in North America. GAT was at the forefront of industry change in the 1960s, innovating and creating patented technology and processes. We’ve always had a strong focus on family and our employees, which is why people come to work for GAT and stay here for many years.